Sometimes I hear people say "I don't go out late on weeknights" when they mean Sunday through Thursday nights. Other times "weeknights" refers to Monday through Friday nights. Is there a less ambiguous way to say the former?
You can use worknight (or work night):
An evening before a day one has to work; a school night for someone who works rather than goes to school; typically the evenings of Sunday to Thursday.
So someone could say, "I don't stay out late on worknights" to refer to the evenings before a workday.